A Systematic Approach to ECG
Interpretation
Introduction
This guide provides a structured method for interpreting a 12-lead ECG.
Initial Checks
Before interpreting an ECG, confirm the following:
Patient’s name and date of birth match the ECG details.
Date and time the ECG was performed.
ECG calibration (usually 25 mm/s speed and 10 mm/mV amplitude).
Heart Rate
Normal Ranges
Normal: 60–100 bpm
Tachycardia: >100 bpm
Bradycardia: <60 bpm
Rate Calculation: Regular Rhythm
1. Count the number of large squares between two R waves.
2. Divide 300 by this number.
Example: 4 large squares → 300 ÷ 4 = 75 bpm
Rate Calculation: Irregular Rhythm
1. Count the number of QRS complexes in a 10-second rhythm strip.
2. Multiply by 6 to estimate bpm.
Example: 10 complexes → 10 × 6 = 60 bpm
Heart Rhythm
Types of Rhythm
, Regular
Irregular:
o Regularly irregular (patterned irregularity)
o Irregularly irregular (no pattern)
Use a piece of paper to mark several R-R intervals and slide it along the rhythm strip to
assess regularity.
Tip: If atrioventricular (AV) block is suspected, map out P and R waves separately to assess
PR interval changes and QRS dropouts.
Cardiac Axis
Cardiac axis describes the general direction of electrical activity in the heart.
Normal Axis
Leads I and II are positive.
Lead II shows the tallest QRS complex.
Right Axis Deviation
Lead III is more positive than Lead I.
Associated with right ventricular hypertrophy.
Left Axis Deviation
Lead I is positive; leads II and III are negative.
Often due to conduction abnormalities.
P Waves
Check the following:
1. Are P waves present?
2. Does each P wave precede a QRS complex?
3. Are P waves normal in shape, duration, and direction?
4. If absent, is there:
o Sawtooth baseline → atrial flutter
o Chaotic baseline → atrial fibrillation
o Flat baseline → no atrial activity
Tip: Irregular rhythm without visible P waves suggests atrial fibrillation.
Interpretation
Introduction
This guide provides a structured method for interpreting a 12-lead ECG.
Initial Checks
Before interpreting an ECG, confirm the following:
Patient’s name and date of birth match the ECG details.
Date and time the ECG was performed.
ECG calibration (usually 25 mm/s speed and 10 mm/mV amplitude).
Heart Rate
Normal Ranges
Normal: 60–100 bpm
Tachycardia: >100 bpm
Bradycardia: <60 bpm
Rate Calculation: Regular Rhythm
1. Count the number of large squares between two R waves.
2. Divide 300 by this number.
Example: 4 large squares → 300 ÷ 4 = 75 bpm
Rate Calculation: Irregular Rhythm
1. Count the number of QRS complexes in a 10-second rhythm strip.
2. Multiply by 6 to estimate bpm.
Example: 10 complexes → 10 × 6 = 60 bpm
Heart Rhythm
Types of Rhythm
, Regular
Irregular:
o Regularly irregular (patterned irregularity)
o Irregularly irregular (no pattern)
Use a piece of paper to mark several R-R intervals and slide it along the rhythm strip to
assess regularity.
Tip: If atrioventricular (AV) block is suspected, map out P and R waves separately to assess
PR interval changes and QRS dropouts.
Cardiac Axis
Cardiac axis describes the general direction of electrical activity in the heart.
Normal Axis
Leads I and II are positive.
Lead II shows the tallest QRS complex.
Right Axis Deviation
Lead III is more positive than Lead I.
Associated with right ventricular hypertrophy.
Left Axis Deviation
Lead I is positive; leads II and III are negative.
Often due to conduction abnormalities.
P Waves
Check the following:
1. Are P waves present?
2. Does each P wave precede a QRS complex?
3. Are P waves normal in shape, duration, and direction?
4. If absent, is there:
o Sawtooth baseline → atrial flutter
o Chaotic baseline → atrial fibrillation
o Flat baseline → no atrial activity
Tip: Irregular rhythm without visible P waves suggests atrial fibrillation.
